Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A well known class of fuel cells, referred to in the art as “solid-oxide” fuel cells (“SOFC”), includes a solid-oxide electrolyte layer through which oxygen anions migrate from a cathode to combine with hydrogen, forming water at the anode. In an SOFC, electrons flow through an external circuit between the electrodes, doing electrical work in a load in the circuit.
In the prior art, an SOFC is readily fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. An SOFC can use fuel gas containing CO with the H2, the CO being oxidized to CO2.
The long term successful operation of an SOFC depends primarily on maintaining structural and chemical stability of the fuel cell components during steady state conditions, as well as transient operating conditions such as cold startups and emergency shut downs. Three types of reformer technologies are typically employed in conjunction with an SOFC (steam reformers, dry reformers, and partial oxidation reformers) to convert hydrocarbon fuel to hydrogen using water, carbon dioxide, and oxygen, respectively, with byproducts including carbon dioxide and carbon monoxide, accordingly.
Known hydrocarbon fuels for use in a reformer are, for example, gasoline, diesel, JP-8, Jet-A, and natural gas. A serious problem in the use of such fuels can be the presence of sulfur and sulfurous compounds. Ultra-low sulfur road fuels, being introduced in Europe and North America, have low levels of sulfur, with limits in the range of 10 to 50 parts per million (ppm) by weight. Some refinery streams and, for example, Fischer Tropsch synthetic diesel fuel are essentially sulfur-free—but when distributed in the fuel infrastructure it is very difficult to consistently deliver fuels with a sulfur level of less than 30 ppm. In some regions of the world, commercial hydrocarbon fuels contain elevated levels of sulfur, e.g., in an amount of about 300 to about 5,000 ppm by weight. It is likely that these high sulfur fuels will continue to be used in some parts of the world and in some industries (for example shipping and aviation) for long into the future. Fuel cell stacks can be particularly sensitive to sulfur—which tends to accumulate in the anode and cut power density and efficiency. Reformer catalysts and washcoat materials may also have some sensitivity to sulfur. In addition, endothermic reformer catalysts operating at low temperature tend to be particularly intolerant to sulfur, which can also adversely affect achievable reformer efficiency. In addition, sulfur can increase the propensity to form soot and other carbonaceous deposits. If coking or sooting occurs, due to a premature gas phase reaction before the fuel enters the reformer, within the reformer or as a post reaction in the system manifolding, the resulting particulate matter can enter the SOFC and degrade its efficiency and performance. Thus the long term successful operation of the fuel cell system is compromised by sulfur in the fuel.
Pending U.S. patent application, Ser. No. 09/781,687, filed Feb. 12, 2001, published Sep. 26, 2002 as US Patent Application Publication No. 2002/0136936 A1, the relevant disclosure of which is incorporated herein by reference, discloses a system and method for trapping impurities and particulate matter, and especially sulfur and sulfur-containing compounds, in energy conversion devices. The system comprises a regenerable trap including a trap element and, optionally, a filter element. The reforming system is fluidly coupled to the trapping system, which is positioned after the reforming system.
A drawback of the disclosed trappng system is that when the trap becomes loaded with trapped material, fuel cell operation must be suspended in order for the trap to be purged of the trapped material and thus regenerated. During such regeneration, the reformer is operated in a fashion to produce a gas suitable for removal of the trapped material (i.e., at high oxygen/carbon ratios) and the reformate gas is passed through the trap, reversing the adsorption process. The effluent from the trap is exhausted from the system via a control valve. A problem with this approach is that the fuel being reformed during regeneration is still contaminated with sulfur. Another problem is that the temperature at the reformer exit may be more than 900 C during start-up which can deteriorate the active materials in the sulfur trap. Yet another problem is that an extra heat exchanger must be used upstream of the reformer to cool recycled anode gas when the recycled gas is used to provide an oxidant for endothermic reforming.
What is needed in the art is a method and apparatus that permits continuous supply of desulfurized reformate to a fuel cell while simultaneously permitting regeneration of the sulfur strap, in an efficient configuration that protects the active materials in the sulfur trap from high temperature modes.
It is a principal object of the present invention to provide a continuous stream of sulfur-free reformate to a fuel cell for continuous operation thereof.